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First question…

What’s an instance variable?

In the Ruby programming language, an Instance variable is a type of variable which starts with an @ symbol.

Example:

@fruit

An instance variable is used as part of Object-Oriented Programming (OOP) to give objects their own private space to store data.

We say that objects can:

1. Do things
2. Know things

Methods make things happen, they DO things.

Instance variables store data, they KNOW things.

Example:

If you have a Fruit class, you may want to know what kind of fruit it’s, what color, weight, etc.

All of these attributes become instance variables.

Like @color, @type, @weight.

Next:

Let’s see code examples.

How to Define & Use Ruby Instance Variables

You define instance variables inside classes.

Example:

Let’s say we have a CoffeeMachine class.

A coffee machine needs water, so you may want to know how much water is available.

We can do this:

class CoffeeMachine
  def initialize
    @water = 100
  end
end

This @water is our instance variable.

We assign 100 to @water as the initial value.

Now:

If we have a make_coffee method, we can reduce the amount of water left in the tank.

class CoffeeMachine
  def initialize
    @water = 100
  end

  def make_coffee
    @water -= 10
  end
end

Notice that every CoffeeMachine object you create (with new) will have its own private value for @water.

Makes sense, right?

Because if you have 10 actual coffee machines, each is going to have their own water levels.

That’s why we use instance variables!

How to Access Instance Variables

Instance variables wouldn’t be very useful if you couldn’t read their current value.

You can read an instance variable value with the @ syntax.

Like this:

class CoffeeMachine
  def initialize
    @water = 50
  end

  def print_water_level
    puts "Water Level: #{@water}"
  end
end

machine = CoffeeMachine.new
machine.print_water_level

# Water Level: 50

The print_water_level method uses @water to print its value.

Using Attribute Accessors

You may notice that you can’t access instance variables from outside the class.

That’s by design!

It’s what we call “Encapsulation”, an object’s data is protected from the outside world, like other Ruby objects.

Here’s what I mean:

machine = CoffeeMachine.new

machine.water
# NoMethodError: undefined method `water' for #0x2d0a530>

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